Various wireless communication devices such as cellular telephones, radios, and wireless modems require RF amplifiers to amplify the signal received from an antenna or the like. The amplifier must also provide an impedance match to the antenna. Typically, the amplifier is divided into an integrated circuit chip having the transistors and other circuitry associated with the amplifier and a number of off-chip components that filter the signal or provide impedance matching at a particular operating frequency. The integrated circuit amplifiers commonly used have a number of significant problems.
First, the circuits provide good input impedance matching over a relatively small frequency band. In general, the matched band decreases in width as the center frequency of the input band decreases. In principle, this problem can be reduced by utilizing more off-chip matching components. However, such a solution increases the component count, and hence, the manufacturing cost of the amplifier. In addition, the insertion loss imposed by these additional matching components increases, and hence, the net gain of the circuit decreases.
Second, the gain of the amplifier drops rapidly at frequencies outside the matched frequency. Hence, wide band amplifiers are difficult to construct.
Third, the gain of the device typically depends on the gain of a single component. Hence, manufacturing variations in the properties of this component lead to large variations in the gain of the amplifier. In addition, such circuits are prone to instabilities when this single component has high gain.